> There's just something about bipolar junction transistors that I love.
> Possibly because they're so easy to use in power applications controlled
> directly from a 3.3V microcontroller pin.

> I'm looking for a "jellybean" power BJT (preferably in a Darlington
> configuration). Must be available and easy to acquire, decent price and
> performance, and in a human-friendly through-hole package (TO-220 would
> be great).
>
> I was looking at the TIP122 earlier, hoping someone has a better suggesti=
on.

These are not the transistors you are looking for.
MOSFET are.

With BJTs, as power rises you run out of drive.
If you can sink say 20 mA from a pin then the BJT can drive Beta x 20 mA.
Beta falls as Ic rises.
People like Zetex (now part of Diodes Inc) have a special magic potion
that they use to make transistors with better specs in all areas but
even they have limits.
Beta of say 100 at 2A is getting unusual. 20 mA x 100 =3D 2A.
You may be lucky - you MAY get even more but I'd suspect that 4A or so
would be very hard with 20 mA drive.

WHEREAS, once you get a MOSFET with low enough Vgs there is no
sensible upper limit on controlled current. Main issue then is
switching speed. MOSFET gate may be 1 to 5 nF range capacity and at
say 20 kHz + yiu want 100's of mA drive for short periods to get good
transition times.
A very very simple and cheap jelly bean driver will handle this.
Best case it is 2 x small BJT and NOTHING else. Very rude. Works.
Needs even lower Vgs FET.

A Darlington need 2 x Vbe to operate. At least 1.2V and probably 1.5V
and maybe 2V+ as power goes up.
3V3 pin will drive this but some care needed. Direct drive with no R is nau=
ghty.
20 MA with Beta  1000 =3D2 20A.
BUT 20 mA will need to be into maybe 2V base and most uCs do not drive
that high that hard.

I'd find a MOSFET that suits when using a MickeyMouse gate driver and
standardise on it.
Much easier overall.

Mickey Mouse gate driver,

NPN + PNP (BC337/BC327 or BC807/BC817 very good).

Connect bases
Connect emitters
NPN collector to V+
PNP collector to ground.
Dtrive bases with pin
Connect emitters to MOSFET gate.
QED
Very nasty.
Works well.

Shoot though? What's shoot through? :-).
Experiment.
Maybe a few Ohms emitters to gate.
Zener gate to ground if load inductive (or always if wise)(Anode to ground)

MOSFET needs Vth < or << 2V or so.
Once you find one that suits with desired current capacity you have a
friend for life


    Russell McMahon
--=20
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